Spirit level and flash. That's all.
If you put the flash directly there, the light is not good. This for I modified a flash to be fitted verticaly on this rail.
this pichture shows all my "flash" modifications needed for weddings:
Special "rapid" plate; modified Rollei flash for motorcycle's battery; pistol grip for Metz 45; PM5 with TTL-flashmeter and flash to fix on rail camera.
Perhaps not a beauty but it worked for years !
I have other idea that will be realized if I buy a CFV... or CFV II
Now, I'm concerned by my Leica M8.